With the development of multi-media technology, there is an increasing demand for constructing optical communication networks that are capable of providing transmissions at high speeds and accommodating large volumes of information. There is also a demand to provide such transmissions at lowered costs and over long distances. Various conventional systems that provide transmission high rates between 10 Gb/s to over 40 Gb/s have been developed or are currently under development.
To accommodate such high speeds of transmission, electronic circuits having bias-tee (also referred to as “bias-T”) packages disposed on transmitter-receivers have been developed. A conventional bias-tee circuit is in actuality a form of a multiplexer having three ports arranged in the shape of a “T” and having frequencies ranging from below 30 KHz to at least 40 GHz pass horizontally through the T, and combine with much lower frequencies including DC from the bottom path used to bias and/or modulate transistors, diodes, and passive circuits. The circuit is a simple composition of one capacitor and one coil with some attention paid to details.
In the conventional systems, the construction of a horizontal bar of the T is based on one or more of the many forms of transmission line having low-loss, non-conducting material, including gas, act as a dielectric. At one point, a small slice is cut out from the transmission line conductor. Thus, a capacitor is formed and low frequencies are blocked. This kind of capacitor has an advantage that it is nearly invisible to higher frequencies. To pass frequencies of typically several mega-Hertz and lower, the capacitance has to be increased. An ultra-wideband capacitor, such as type 545L capacitor manufactured by American Technical Ceramics Corporation, and disclosed in the co-owned U.S. Pat. No. 7,248,458 to Mruz, the disclosure of which is incorporated herein by reference in its entirety, can be configured to accomplish this task without adding significant perturbations to the insertion loss and return loss characteristics of the original straight-through line.
A small coil made of fine wire with an air core, a dielectric core, ferrite core, or a powdered iron core connects the inner conductor of one of the sides of the capacitor with the port in the outer conductor leading down the T. Frequencies above of approximately 16 KHz hit the coil at the small end. Because of increasing diameters of the coil as the windings progress along the tapered length of the core, its resonances are distributed across the entire frequency range of the T causing its inductive reactance characteristic to vary uniformly with frequency. This results in a virtually resonance-free increase in the inductiveness of the coil, which, in-turn, causes a linear reduction in an RF leakage from the transmission line, as the frequency increases. Because of size constraints, this type of singe layer, tapered coil cannot be made with enough inductance to sufficiently prevent RF leakage that occurs at very low frequencies. Thus, conventional systems implement a second coil, having considerably more inductance, to be placed in-series with the first tapered coil starting at the large end of the first tapered coil.
Any resonances that may result from the larger coil and interactions between these two tandem coils are dampened by two resistors placed across the larger coil and in-series with both coils, respectively.
The conventional bias-tee packages are commonly used for biasing of photodiodes (vacuum and solid state), Microchannel plate detectors, transistors, and triodes. This stops high frequency energy from leaking onto a common power supply rail and stops noise from the power supply from leaking onto the signal line.
The conventional systems employing the bias-tee packages suffer high and erratic insertion loss (i.e., a decrease in transmitted signal power) when operated over a wide range of operational frequencies. Thus, there is a need for a system that can operate in a wide range of frequencies and with low and well-behaved insertion loss.